This invention relates to height or displacement adjustment mechanisms for chairs and similar fixtures, and more particularly relates to mechanisms for adjusting the height of chairs which are designed to swivel without altering the previously adjusted height of the chair seat.
In order to selectively adjust the height of a chair seat, tabletop or similar fixture, relative to the base of the fixture, numerous height adjustment mechanisms have been developed as described for example in U.S. Pat. Nos. 3,161,396; 3,711,054; 3,741,514 and 3,778,014. Since modern office chairs are designed to allow swiveling of the seat without modifying the height of the seat which the user wishes to maintain, the arrangement of the height adjustment mechanism must also enable such swiveling. In the height adjustment mechanisms for swiveling chairs, as described for example in U.S. Pat. Nos. 3,386,697 and 3,799,485, the threaded spindle is rotatably mounted in the chair base to enable swiveling of the height adjustment mechanism and the seat is supported on a column arranged for displacement with a spindle nut which can be threaded along the spindle to either elevate or lower the column and seat when a locking device has been activated to restrain rotation of the spindle. The mechanisms described in both of these patents require an inconvenient height adjustment operation performed by the user who must continuously lift and hold activating device with his foot in order to lock the spindle against rotation throughout the simultaneous operation of manually rotating the seat to accomplish the height adjustment. Both of these mechanisms also require detent interference between the nut and spindle which must be repeatedly overridden by the user's corresponding applications of intermittently increased torque on the seat in order to perform the height adjustment operation and the resulting height adjustment can only be obtained in the increments governed by the number and configuration of the detent formations. Mechanisms incorporating pressurized fluid drive for chair height adjustment have been developed and described, for example in U.S. Pat. Nos. 3,711,054 and 4,113,220. However, these mechanisms have been subject to failure in pressure seals and gas leakage. In the effort to improve the convenience of the height adjustment operation, the mechanism described in U.S. Pat. No. 4,493,469 incorporates a threaded screw and nut arrangement in which the screw and nut can be locked and unlocked using a system of locking bushings and a linkage therefor which is manually activated and deactivated. However, this mechanism requires a particularly large number of cooperating, movable components and allows height adjustment only in predetermined increments governed by the configuration of a locking bushing.
In the further effort to simplify the chair height adjustment operation, U.S. Pat. No. 4,261,540 describes a height adjustment mechanism in which height adjustment is automatically performed whenever the seat is unoccupied and rotated; the mechanism allows automatic disengagement of the height adjustment drive component whenever the seat is occupied, or otherwise weighted, so that the seat can be swiveled without altering the height. However, the automatic activation of the height adjustment drive produces undesired height adjustment whenever the unoccupied chair seat is rotated, for example when the user may be maneuvering the radial orientation of the unoccupied seat or relocating the entire chair. This mechanism does not include a rotatable screw and nut arrangement, but provides an entirely stationary, threaded sheet metal sleeve secured to the interior of the support column in the base of the chair; a threaded member is exteriorly threaded within the sheet metal liner to provide vertical adjustment of an unthreaded chair supporting spindle (shaft) to which the threaded member is releasably locked by a spring biased detent means when the chair is unoccupied. The unthreaded shaft is supported on the biasing spring so that when the chair is occupied the spring is compressed to automatically release the detent connection to the threaded member and the shaft swivels without height adjustment upon the compressed spring.